Many industrial and mobile applications use hydraulic or pneumatic cylinders to control the movement and position of machinery. Precise control of a piston's position within a cylinder is often fundamental to controlling the machinery's operation. Accordingly, industry has produced various mechanical, magnetic, acoustic, and optical techniques for detecting the instantaneous position of a moving piston with respect to the cylinder.
One technique is to encode position information magnetically in the material of the piston rod. Typically, the substrate of the piston rod is made of a ferromagnetic material, such as steel, and can be magnetized. However, such material is magnetically “soft.” In general, magnetically soft material has low coercivity, the measure of difficulty for magnetically encoding and erasing information in that material. Thus, position information encoded in material with low coercivity is subject to accidental erasure or alteration.
To improve information retention, various position detection systems plate the magnetically soft piston rod with a magnetically hard recording medium with high coercivity, and record the position information in this recording medium. In addition, the magnetically hard recording medium can have chrome plating for protection from the harsh operating conditions within which the piston operates, internal conditions, such as the pressurized fluid that drives the motion of the piston, and external conditions, such as dust and debris.
Most magnetic recording is longitudinal in orientation. With longitudinal recording, magnetization lies in the plane of the recording medium. The magnetic pattern of longitudinal recording media consists of transitions or reversals of the in-plane magnetization from one polarity to the other. The magnetic poles, whose stray flux is sensed by a read head located above the recording medium, mark the reversals. Longitudinal recording, however, does not perform well in the presence of a ferromagnetic (i.e., magnetically soft) substrate. Being magnetically permeable, a ferromagnetic substrate provides a low reluctance path for the magnetic flux. This low reluctance path can make longitudinal recording difficult to achieve because it is challenging to maintain a uniform magnetic field parallel to the substrate surface. Additionally, the magnetically permeable substrate can cause the longitudinal encoding to exhibit a reduced level of magnetic flux available for sensing, thus posing a problem for information retention and readback.